1. Field of the Disclosure
This disclosure relates to a powerful fiber laser system. In particular, the disclosure relates to at least one gain block based on a multi-clad multi-mode (MM) active fiber with a core, which is configured to support a fundamental mode without coupling to higher modes, and a pump assembly provided with a plurality of pump channels each of which includes a plurality of single-mode (SM) fiber lasers coupled to a SM-MM combiner which has a low mode output coupled to the gain block.
2. Background of the Disclosure
The prior art powerful fiber laser systems known to applicants are limited in power. However, the areas including, among others, laser material processing, military, printing, cutting, marking and drilling are in need for fiber laser systems delivering higher than presently available output powers while generating a high quality light beam.
FIG. 1 illustrates a general schematic view of a multi-cascaded fiber laser system 10 of the known prior art while 1B illustrates system 10 shown with pumping assemblies. The system 10 is capable of delivering a SM output power of up to about 2-3 KW—one of the highest among known systems—while generating an optical output in a 1 micron (μm) band. The multi-cascaded system 10 includes an ytterbium (Yb) fiber oscillator 12 and multiple Yb fiber amplifiers 14 (only one is shown) coupled in series to one another by respective passive photosensitive fibers 13. Adjacent photosensitive SM fibers 13 have respective HR fiber grating 16′ and fiber grating 16″ defining a laser cavity which receives oscillator 12 and amplifier 14. The oscillator 12 and amplifiers 14 are configured with respective Yb-doped double clad fibers, each of which is pumped by a plurality of combined 25 W MM laser diodes 18. The pump light from each plurality of MM diodes 18 is launched into the inner cladding of respective Yb fibers 12, 14 by a dedicated MM-MM combiner 20. A system output signal is propagated through a single mode (SM) delivery fiber 13. The system 10, while being robust, compact and enjoying a well-deserved commercial success, is limited in power for the following reasons.
Providing additional cascades, each of which is pumped by a dedicated pump assembly, and/or increasing a pumping power of each existing pump assembly, theoretically, can lead to greater powers of system 10. However, neither of these solutions may be effective, as explained below.
Adding amplifying cascades in excess of three leads to the increased length of system 10. One of ordinary skills in the laser art readily understands that with the increased length of system 10, the non-linear effects, which restrict the efficiency and operability of the system, also increase.
For example, one of these non-linear effects arises from stimulated Raman scattering (SRS). The Raman effect allows for much of the pump energy to be transferred to light at the lower frequency, called the Stokes component. In other words, the SRS involves a type of resonance resulting in generating new wavelengths of light. In certain situations, this phenomenon is of a great positive importance; in others, as here when system 25 operates in a 1μ band, it is a detriment because at some of these Raman-generated wavelengths, the laser power is saturated. Thus, even if the greater pump powers were available, they would not translate into a precipitously greater output and, thus, render system 10 inefficient. A solution to this particular problem includes increasing a wavelength at which the fibers operate.
Furthermore, the higher concentration of rare earth elements, the more efficient Yb fibers 12-14. However, as a rule, during doping, a small amount (a few ppm) of impurities is also introduced into active fibers. At the fiber lengths of about 30-40 meters, these impurities are responsible for up to one (1) dB of losses at the desired laser wavelength. At this dB level, adding new cascades makes no sense since the amplifiers are saturated. Hence, system 10 becomes inefficient.
Increasing the pump power of each pump assembly above presently available is also problematic. The system 10 requires that the pump light be generated at a wavelength of about 970-980 nm to operate at the desired lasing wavelengths. To meet this requirement, system 10 utilizes relatively powerful MM 20-25 W diodes 18, the use of which poses serious problems preventing higher than presently available pump powers for the following reasons.
To begin with, applicants are unaware of MM diodes more powerful than currently used in system 10 for the desired wavelength. Furthermore, even if more powerful 970-980 nm MM diodes were available, they would not solve at least some of further problems associated with fiber system 10, as discussed below.
One of these problems is excessive heat generated by high pumping powers. Currently, for example, nineteen (19) pumping diodes 18 (thirty eight (38) bidirectionally), coupled to each of Yb-doped fibers 12, 14, generate a heat of about 250 W in each cascade. Even with the most sophisticated heat reducing efforts, temperatures still do not fall far below 100° C. in each cascade of system 10. Accordingly, if more powerful MM diodes were available, the heat problem would be even more severe. To reduce the heat generation, as readily understood by one of ordinary skills in the laser art, the difference between a pump wavelength (Lp) and a lasing emission wavelength (Le), at which active fibers 12-16 operate, should be minimized.
Even if the elevated temperatures were kept under control, the possibility of combining together more than nineteen MM diodes 18, for example thirty seven or more diodes, by existing MM-MM combiners 20 would be technologically challenging. The overall diameter of the combiner's output 21, which guides light from MM-MM combiner 20 to an inner cladding of a respective one of fibers 12, 14, increases with the number of diodes 18. However, lightguide 21 coupled to the pump input of active fiber 12, 14 should remain as small as possible for the reasons explained immediately below.
In accordance with a well known side pumping technique, which is preferably used in system 10, a doped core of each of Yb-doped oscillator and amplifier 12, 14, respectively, can effectively absorb pump light energy delivered by lightguide 21 along a certain coupling length. When the diameter of lightguide 21 increases with a greater number of MM diodes 18, the coupling length should be increased in order to effectively absorb the light delivered by output lightguide 21. The increased coupling length is associated with increasing non-active losses in rare-earth-doped fibers.
One of possible solutions to the above-discussed problem is to reduce the diameter of lightguide 21. However, this may be impossible for the following reason. Each of MM diode 18 has a large numerical aperture (NA). To couple light emitted by multiple MM diodes 18 to a pump input 21 of oscillator 12, for example, the latter has to have a NA large enough to receive the light from combined diodes 18. This, in turn, is associated with greater fiber lengths to effectively couple light propagating through lightguide 21 to respective Yb-doped fibers 12, 14. As discussed above, the greater lengths are highly undesirable because of non-linear effects. A solution to these problems lies in SM high pump power sources which are combined in a SM-MM combiner having its output lightguide 21 minimally sized. The core of the lightguide should be dimensioned to provide for minimal ratio between the area A1 of the core of Yb fiber 12 to the area A2 of the entire 8-shaped configuration of FIGS. 2A and 2B including the sum of the areas of cladding of Yb fiber 12 and lightguide 21, respectively.
Furthermore, it is highly desirable to have a diffraction-limited (bright) pump light. To meet this need, SM pump fiber diodes combined by a SM-MM combiner should be used. However, at the desired wavelengths, SM pump diodes are not powerful. Accordingly, the pump light generated by presently used MM diodes combined by MM-MM combiner 20 is far from having a high quality beam.
As to the wavelength, light emitted by system 10 in a range of about 1.06-1.0.8 μm, when scattered, may not be completely eye-safe. Thus, manufacturing facilities and industrial sites associated with an operation of 1 μm powerful laser systems may be confronted with expenses stemming from additional safety measures.
Also, as known, a signal, emitted at about 1-1.4 μm at long distances of up to kilometers, experiences substantial hydrogen-induced losses. Thus, to effectively utilize powerful lasers in applications involving, for example, a subterranean drilling or military operations, an optical signal, preferably, should be emitted at a wavelength of about 1.4-1.6 μm known for minimal hydrogen-induced power losses.
To obviate the eye-hazardous situation, it is possible to substitute Yb-doped fibers 12, 14 for Yb/Er-doped fibers in system 10 which emits light in an eye-safe 1.5 μm band. However, at high pump powers, Yb/Er fibers are associated with the appearance of color centers which tend to extend into a UV range. This phenomenon is critically detrimental to a lightguide, which rapidly degrades. Furthermore, the Yb/Er fiber systems are associated with excessively elevated temperatures affecting the operability of active fibers.
It is, therefore, desirable to provide a powerful fiber laser system operative to deliver a power of at least 10 kW and, preferably, about 20 kW.
It is further desirable to provide a powerful fiber laser system with a pumping assembly, which comprises a plurality of SM fiber pumps combined by a SM-MM combiner so as to launch a high beam quality pump light into one of the inner claddings of an LMA multi-clad multimode (LMA MC&MM) active fiber, which is configured with a core capable of supporting a fundamental mode without mode distortion
It is further desirable to reduce heat generation in a powerful fiber laser system including at least one LMA MC&MM rare-earth doped fiber, which is operative to lase a signal output at a first wavelength (Le), and a pump source operative to emit an optical pump output at a second wavelength (Lp), wherein the Le/Lp is less than 0.05 Lp.
It is further desirable to provide a multi-cascaded powerful fiber laser system including a plurality of LMA MC&MM rare-earth-doped active fibers, each of which is configured with a core capable of supporting a fundamental mode, respective signal SM photosensitive fibers mode distortedly coupled to and alternating with the active fibers, and respective pumping assemblies, each of which includes a plurality of SM fiber lasers combined by a SM-MM fiber combiner so as to launch a pump light in the MM inner cladding of the active fiber.
It is further desirable to provide a multi-cascaded powerful fiber laser system configured with an LMA MC&MM Er-doped oscillator and at least one similarly configured fiber amplifier or booster each pumped by a dedicated pump assembly, which includes a plurality of SM Raman fiber pumps combined by a SM-MM combiner so as to deliver a high beam quality pump light to the inner cladding of a respective one of Er-doped oscillator and at least one amplifier or booster.
It is further desirable to provide a multi-cascaded powerful fiber laser system including an LMA MC&MM Er-doped fiber oscillator and at least one LMA MC&MM Er-doped fiber amplifier or booster each pumped by a dedicated pumping assembly, which is configured with a plurality of SM Yb/Er-doped fiber lasers combined by a SM-MM combiner so as to deliver a high-quality low mode—up to 10 different modes—light beam to the MM cladding of a respective one of Er-doped oscillator and at least one amplifier.
It is further desirable to provide a multi-cascaded fiber laser system with an LAM MC&MM Tm oscillator and at least one similarly configured Tm-doped fiber amplifier, wherein the MM cladding of each Tm-doped component receives a pump light from a respective pumping assembly which has a plurality of SM Er-doped lasers combined by a SM-MM combiner.
It is further desirable to provide a powerful fiber laser system operating in an eye-safe wavelength band.
It is further desirable to provide a powerful multi-cascaded fiber laser system including a plurality of LAM MC&MM Yb-doped fiber oscillator and an at least one Yb-doped fiber amplifier, each of which has a core configured to support a fundamental mode, and a plurality of fiber pumping assemblies each including multiple SM Nd-doped fiber lasers which are combined by a SM-MM combiner so as to lunch a pump light in the MM cladding of the Yb-doped active fiber component.
It is further desirable to provide a powerful multi-cascaded fiber laser system including a plurality of LAM MC&MM Yb-doped fiber oscillator and an at least one Yb-doped fiber amplifier each of which has a core configured to support a fundamental mode, and a plurality of fiber pumping assemblies each including multiple SM Yb-doped fiber lasers which are combined together by a SM-MM combiner so as to lunch a pump light in the MM cladding of the Yb-doped active fiber component.